Not all wine needs to go through malolactic fermentation (MLF) and one of the objectives of MLF is to reduce acidity. That is achieved by relying on malolactic bacteria to convert L-malic acid into L-lactic acid. Apart from de-acidification, MLF may also influence the organoleptic characters of wine.
There are a number of factors which are important to a smooth execution of MLF, namely pH, SO2, temperature and the level of alcohol. Other than these factors, polyphenols and pesticide residues from the vineyards also influence the development of lactic acid bacteria. If we describe malolactic bacteria as workers, they need to be fed to complete their work. Therefore, the nutritional components in the must need to be sufficient. Oenococcus oeni requires a diverse range of nutrients to sustain its growth. It is true that fructose, glucose and pentoses are available in the must as they are a source of sugar. Amino acids and peptides provide nitrogen to the bacteria but Oenococcus oeni needs vitamins and other trace elements such as manganese, potassium and magnesium.
What is important is to understand the fermentation yeast strain. If the yeast strain demands a high level of nutrients and the musts have a low level of nutrients, high level of SO2 can be produced which is strongly inhibitory to malolactic bacteria. The solution is to inoculate a suitable malolactic starter culture with an appropriate bacteria nutrients at the right moment during the MLF. When MLF is struggling, add a specific nutrient. Closely monitor the MLF.
到課室學習品酒的朋友,不時會聽到MLF這三個英文字. 有人會說這是第二次的發酵. MLF是代表malolactic fermentation, 過程是把L-malic acid化為L-lactic acid和CO2. 要順利進行要靠一種稱為oenococcus oeni的乳酸菌. MLF確實是在紅酒或白酒釀製時發生的. 在把果酸變為乳酸時,酒的酸度會降低,但同時也影響了酒中的味道,很多時是令酒味更複雜. 但釀酒師最頭痛是MLF不能順利完成. 換言之,問題是果酸轉乳酸的真菌是否能正常有效率地工作.
影響這個過程的其中關鍵是酒中的酸度pH, 酒精濃度,二氧化硫和溫度. 此外,酒中是否含有足夠的養份供給真菌也是重要的因素. Oenococcus oeni 首先最需要的養份是果糖 fructose,五碳糖 pentoses ,葡萄糖 glucose,氮 nitrogen例如是氨基酸 amino acids或縮氨酸 peptides. 維他命尤其是B-group和泛酸維他命 pantothenic acid. 其他的養份還有鎂 magnesium, 鉀 potassium 和錳 manganese. 只要是主要的養份不足, 尤其是釀酒的環境欠佳的話,MLF便會出現問題.
有些酒廠在酒進行發酵時選用一些人工酵母菌,而這類菌又是在營養要求上特別高的話,到進行MLF時便會影響oenococcus oeni的效能. 有些葡萄的汁液本身的營養份不高,酵母菌會有可能在發酵時製造高度的SO2, 這會影響果酸轉乳酸的真菌的工作能力. 想MLF進行順利可以因應所用的酵母菌種類而選擇合適的果酸轉乳酸的真菌,同時要配合使用適當的營養補品,小心監察進度,便可避免stuck MLF 帶來的問題,包括Dekkera/Brettanomyces的真菌滋生.